1. Field of the Invention
The invention relates to a window blind, more particularly to a window blind having an operating device for concealed pull ropes thereof.
2. Description of the Related Art
Referring to FIGS. 1 and 2, a conventional window blind includes an elongated top housing 10, a horizontal shaft 11 journalled in the top housing 10, a plurality of slats 14 suspended one above another from the top housing 10, a bottom rail 16 disposed below the slats 14, a pair of pull ropes 141, a pair of ladder cords 13, and an operating rod 20. Each of the pull ropes 141 has an anchor end that passes through the top housing 10 and through the slats 14 and that is mounted to the bottom rail 16. Each of the ladder cords 13 is disposed on opposite longitudinal sides of the slats 14, and has an upper end that is secured to the shaft 11 and a lower end that is mounted on the bottom rail 16. The operating rod 20 is coupled to the horizontal shaft 16, permits extension of operating ends of the pull ropes 21 therethrough, includes a pair of retaining members 201, and is operable so as to actuate axial rotation of the horizontal shaft 11, thereby controlling tilting of the slats 14. The retaining members 201 are mounted on an outer surface of the operating rod 20 one above the other. The operating ends of the pull ropes 21, that extend out of the operating rod 20 when the slats 14 are raised, are wound around both of the retaining members 201 and are hooked on one of the retaining members 201. The conventional window blind achieves the purpose of preventing access to the operating ends of the pull ropes 21 by children, thereby avoiding danger in view of possible entanglement. However, the process of winding the operating ends of the pull ropes 21, that extend out of the operating rod 20 when the slats 14 are raised, causes inconvenience on the part of the user.
Therefore, the object of the present invention is to provide a window blind that has an operating device which can overcome the aforesaid drawback of the prior art.
According to the present invention, a window blind comprises an elongated top housing, first and second horizontal shafts, a plurality of horizontal slats, a bottom rail, a pair of pull ropes, a pair of ladder cords, and an operating device. Each of the first and second horizontal shafts is journalled within the top housing. The horizontal slats are suspended one above another from the top housing. The bottom rail is disposed below the slats. Each of the pull ropes extends through the top housing and the slats, and has an upper end connected to the first horizontal shaft and a lower end connected to the bottom rail. Each of the ladder cords is connected to the slats, and has an upper end connected to the second horizontal shaft and a lower end connected to the bottom rail. The operating device includes a hollow frame, a drive bobbin, a drive cord, a spiral spring, a clutch unit, a braking unit, a rotary tilt control unit, a positioning unit, and a pulling unit. The hollow frame is mounted on the top housing and has an innerwall surface. The drive bobbin is disposed rotatably in the frame and is coaxial with the first horizontal shaft. The drive cord has an upper end that is connected to the drive bobbin, and a lower end that extends out of the frame and through the top housing. The spiral spring is wound on the drive bobbin, and has opposite ends connected to the drive bobbin and the frame, respectively. The spiral spring provides a biasing force for biasing the drive bobbin to rotate in a first direction for winding the drive cord on the drive bobbin. The clutch unit includes a driven hub that is disposed rotatably in the frame and that is sleeved securely on the first horizontal shaft, and a coupling spring unit that is sleeved fittingly on the drive bobbin and the driven hub. The coupling spring unit is in friction engagement with the inner wall surface of the frame. The braking unit is disposed in the frame, and includes a braking member and a releasing cord. The braking member provides a braking force that acts on the driven hub when the driven hub tends to rotate in the first direction due to the weight of the bottom rail transmitted to the first horizontal shaft through the pull ropes. The releasing cord has an upper end that is connected to the braking member, and a lower end that extends out of the frame and through the top housing. The releasing cord is operable so as to enable the braking member to release the driven hub from the braking force. The rotary tilt control unit includes a tubular connector that is coupled to the second horizontal shaft. The tubular connector is axially rotatable relative to the frame such that axial rotation of the tubular connector results in corresponding axial rotation of the second horizontal shaft to adjust tilting angles of the slats. The tubular connector extends out of the top housing, and permits extension of the lower ends of the drive and releasing cords therethrough. The positioning unit is coupled to the tilt control unit. The positioning unit permits extension of the lower ends of the drive and releasing cords therethrough, and is connected to the lower end of the releasing cord. The pulling unit permits extension of the lower end of the drive cord thereinto. The pulling unit is connected to the lower end of the drive cord. Rotation of the drive bobbin in a second direction opposite to the first direction due to pulling action applied on the pulling unit enables the coupling spring unit to contract in radial inward directions in order to transmit rotation of the drive bobbin to the driven hub against biasing action of the spiral spring so that the driven hub rotates with the drive bobbin in the second direction, thereby resulting in corresponding rotation of the first horizontal shaft in the second direction for winding the pull ropes thereon and for raising the bottom rail toward the top housing. Rotation of the drive bobbin in the first direction due to restoring action of the spiral spring when the pulling unit is released from the pulling action enables the coupling spring unit to expand in radial outward directions so that the driven hub does not rotate with the drive bobbin. The braking force provided by the braking member is released upon operating the releasing cord so as to lower the bottom rail, and is sufficient so as to support the weight of the bottom rail and the weight of the slats that acts on the bottom rail in order to retain the bottom rail at a desired vertical distance relative to the top housing.